Type 1 diabetes (T1D), the most common childhood autoimmune disease, is caused by the T lymphocyte-mediated destruction of insulin-producing pancreatic beta cells. The salient immunological features of T1D are well-modeled in the non-obese diabetic (NOD) mouse, which like human T1D patients exhibit spontaneous autoimmune diabetes mediated by both CD4+ and CD8+ T cells. While these diabetogenic, or disease-causing, T cells are indispensable cellular participants, their activation and functional maturation are controlled by the innate antigen presenting cells (APC) which acquire, process and present pancreatic beta cell antigen in a manner that breaks normal peripheral T cell tolerance. The APC responsible for breaking T cell tolerance and the molecular mechanism involved is not well understood. It is well appreciated that dendritic cells (DC) are crucial for presenting foreign antigen to T cells during times of inflammation. Yet, the clearance of apoptotic self cells by DC is generally considered to be non-inflammatory, even tolerogenic; in fact, numerous studies have shown that DC that capture antigen from apoptotic cells migrate to local lymph nodes, where they induce T cell tolerance, anergy or deletion. Yet, under certain situations phagocytosis of apoptotic cells by DC can be pro-inflammatory and lead to the priming of self-reactive T cells; this is thought to be the case for the presentation of apoptotic pancreatic beta cells in T1D-prone individuals or animals. Recently, we found that a subset of conventional DC (cDC) are necessary for priming diabetogenic CD4+ T cells in vivo. Interestingly, within this cDC population, our colleague and dual-PI applicant, Dr. Edith Janssen, described a novel DC subset (CD11c+CD11blo/-CD4-CD81-PDCA-1-) that in contrast to other cDC subsets potently (cross-)primes both CD4+ and CD8+ T cells to self-antigen from apoptotic cells. These cells are termed merocytic DC (mcDC) since they stored particles of apoptotic cells in discrete, punctate vesicles in their cytoplasm, meros (<5A?C, particle, in Greek). For example, CD8+ T cells that encounter mcDC exposed to apoptotic antigen are not rendered tolerant, but rather display enhanced primary clonal expansion, cytokine production and effector function. Moreover, mcDC enhance inflammatory immunity to apoptotic self in part by producing copious levels of type 1 interferons. Rationale: Our recent studies have found that mcDC are not only more numerous but also more biologically active in NOD mice compared to other strains. Islet antigen-loaded mcDC (i) rescue CD8+ T cells from peripheral anergy and deletion, (ii) stimulate islet-reactive CD4+ T cells, and (iii) transfer diabetes to young NOD recipients. Moreover, when purified from the pancreatic lymph nodes of overtly diabetic NOD mice, mcDC can break peripheral T cell tolerance to beta cell antigens in vivo and induce rapid onset T cell-mediated T1D in young NOD mouse. Thus, the mcDC subset appears to represent the long-sought critical antigen presenting cell responsible for breaking peripheral tolerance to beta cell antigen in vivo. Hypothesis: Together, these results lead us to hypothesize that mcDC are (i) the critical CD11c+ DC subset responsible for the priming of diabetogenic T cells and cause the functional break in immune tolerance to islet antigen, and (ii) a major reservoir of type 1 interferon needed to drive T1D in NOD mice. To provide a direct test of these hypotheses, we propose the following specific aims: Aim 1: To identify the means by which islet-antigen-fed mcDC break CD4+ and CD8+ T cell tolerance in the NOD mouse. Preliminary data shows that the acquisition and presentation of self-antigen from apoptotic beta cells breaks peripheral CD4+ and CD8+ T cell tolerance in vivo. Aim 2: To define the molecular role of type 1 interferon production by mcDC in the development of T1D in the NOD mouse. PUBLIC HEALTH RELEVANCE: This application focuses on Type 1 diabetes (T1D), the most common childhood autoimmune disease, is caused by the T lymphocyte-mediated destruction of insulin-producing pancreatic beta cells. The salient immunological features of T1D are well-modeled in the non-obese diabetic (NOD) mouse, which like human T1D patients exhibit spontaneous autoimmune diabetes mediated by both CD4+ and CD8+ T cells. While these diabetogenic, or disease-causing, T cells are indispensable cellular participants, their activation and functional maturation are controlled by the innate antigen presenting cells (APC) which acquire, process and present pancreatic beta cell antigen in a manner that breaks normal peripheral T cell tolerance. The APC responsible for breaking T cell tolerance and the molecular mechanism involved is not well understood. Recently, we found that a subset of conventional DC (cDC) are necessary for priming diabetogenic CD4+ T cells in vivo. Interestingly, within this cDC population, our colleague and dual-PI applicant, Dr. Edith Janssen, described a novel DC subset termed merocytic DC (mcDC) which store particles of apoptotic cells in discrete, punctate vesicles in their cytoplasm. Additionally, mcDC enhance inflammatory immunity to self tissue in part by producing copious levels of type 1 interferons. Our recent studies have found that mcDC are not only more numerous but also more biologically active in NOD mice compared to other strains. We hypothesize that mcDC are (i) the critical CD11c+ DC subset responsible for the priming of diabetogenic T cells and cause the functional break in immune tolerance to islet antigen, and (ii) a major reservoir of type 1 interferon needed to drive T1D in NOD mice.